Electrical measuring device



July 13, 1937. F. H. SHEPARD, JR

ELECTRICAL MEASURING DEVICE Filed May 16, 1936 INVENTOR I FRANCIS H.SHEPARD,JR. @44%, ATTORNEY y Patented July 13, 1937 nire , ELECT r Tur Price Francis H. Shepard, Jr., Rutherford, N. Ji., as-

signor, by inesne assignments, to Radio Cor!- `aeration or Anumerica, New York, N. Y.,' a corporation of Delaware .instituciones is, ross, serial No. sans This invention relates to current measuring tremely small values of current in high impedance circuits.

Instruments for testing high impedance cir;

cuits carrying small currents usually comprise a therinionic relay and a milliammeter connected in the output oi the relay with the input electrocles of the relay usually connected across the high impedance to be tested. When the impedance has a Value of the order of several inegohms, leakage' resistances between parts of the circuit of the instrument become comparable to the impedance under test and variation oi supply voltages or tube and circuit characteristics appreciably edect the calibration oi the insi, rvi-l mont. v

It is an object oi my invention to provide a current measuring device adapted to measure eirtremely high impedanccs 'and small currents and which is insensitive to substantial changes in cir cuit constants in the device.

it is a further object of my invention to provide a current measuring device which is rela` tively insensitive to substantial changes in current supply voltages or tube characteristics.

My invention is specically deined in the appended claims and one embodiment is described in the following specication in connection `yith the accompanying drawing in which:

Figure i shows schematically the circuits of one embodiment of my invention, and

iigure 2 shows a group oi curves explanatory of the omration or my device.

The current measuring device, according to one embodiment of my invention and as shown in Figure 1, comprises a thermionic relay tube such as a highly evacuated electron discharge device -i with cathode i, grid t3 and anode t, with the anode connected through the sliding contact of potentiorneter t to the 4positive end oi the voltage source ii, and with cathode i connected to the negative end of the voltagesource t through a to one input or test terminal 9, the other end ci the impedance being connected to the other test terminal iii which may be connected to any point of a potential equal to or higher than the ductance ii is connected between the anode and' and terminals t and it are open circuited grid sumcient to reduce the space current oi the diss' This controlling action of grid t by the-potentiai potential oi cathode ii and may conveniently he connected to the positive end of voltage source t. 'lio obtain an indication oi the voltage drop across resistance "i, l prefer to use a grid controlled gaseous discharge tube, such as a, nas s triode ii with cathode it, grid it! and anode it, with the anode connected through a current meter it, such: as a milliammeter, to the voltage source t and with the grid and cathode connected respectively to opposite en'ds oi resistor i, a ballastjl or current limiting resistor it being connected in series with the grid, and inductance it and potentiometer it being connected in series with the cathode. Condenser it in series ivitl'i incathode oi the gas discharge device, so that discharges in the gas tube cause tient current to now through inductance i'i and condenser it and produce transient changes in voltage at point 2i. Corresponding voltage change oi contact di on the potentiometer may be impressed upon grid ii oi the electron discharge device through con kdenser itil.

When the circuits oi the device are energized,

tl is' free and Will assume a negative charrue 25 charge device i to zero or some low value. The potential of grid i becomes suciently positive to cause appreciable space current flows through the tube and resistor i when the resistance Re is connected between terminals t and iii, and the upper end of resistor i becomes positive. Grid it of the gas triade tube may thus be made suiciently positive to ignite or start a gaseous discharge between the anode and cathode. or the gas tube. Upon the ignition of the gaseous'discharge the energy stored in condenser it by the battery t discharges through the gas tube 'and the series induotance il thereby swinging point 22' at the upper end oi the potentiometer sharply positive. The sudden rise of potential also makes contact il positive, thus impressing upon the lower plate of condenser it a positive charge and drives grid Si oirelay i to a potential at which a considerable now of negative electrons is drawn to the grid, but only continues while contact it is rising in potential. as the potential of rthe contact il decreases from its maximum positive value thev potential of grid ii 50 also decreases and the grid assumes a. negative charge proportional to the decrease of voltage of the contact, the potential of the cathode 2 being held substantially constant by condenser i3..

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of contact 2l appears to be due to the fact that grid 3 is coupled to the contact only through series condenser 20 and that grid 3 responds only to dynamic changes of potential at 2l and is independent of the absolute voltage at 2i, or of contact potential diierences between grid 3 and cathode. As grid 3 of the electron discharge device swings negative, the plate current through tube l and the Voltage across resistor 'I drops to such a value that grid i3 regains control of the gas discharge tube and interrupts the gas discharge.

The time required for the negative charge upon grid 3 to leak off through impedance Rx depends upon the value oi Rx, and the D. C. voltage across RX, and accordingly determines the time required for plate current through tube i and resistor l to return to such a value as to re-ignite a second or another gaseous discharge in gas tube it. The current meter l5 in the anode-cathode circuit of the gas tube integrates the pulses oi current caused by the succession of gaseous dis-v charges and hence gives an indication of the value oi Rx. f

Il'he sequence ofoperations in my improved current measuring device may be graphically represented by the group of voltage vs. time curves shown in Figure 2, in which the potential or" cathode 2 at the instant the gas tube is'ignited has arbitrarily been selected as the zero cr reference potential, and curve 30 has been plotted to indicate voltage variations of grid fi of the relay tube and curve ill plotted to indicate voltage variations of contact 2l throughout two cycles of operation. With the anode potential ci tube l properly adjusted by the sliding contact of potentiometer 5 and with the value of resistance l so selected that the gas tube is ignited at the potential at which grid 3 just starts to draw negative electrons, commonly called grid current, a rise of control grid voltage 3 to point a immediately ignites the gas tube, swinging the potential of contact 2| to some high value b. The sudden rise of voltage of contact 2i yfrom a to b 'urges the control grid 3, through condenser 20, toward a positive Value, but due to the flow of negative electrons to the grid the grid potential continues during this transitory state at substantially cathode or Zero potential, and the collapse o f the potential of contact 2l to zero draws grid 3 to a negative potential with respect to cathode proportional to the potential change of contact 2|,

modified by grid-to-ground capacity, indicated at c. Current ow through RX proceeds t0 raise the potential of grid 3 from c to point a', whereupon the gas tube is re-ignited and the cycle of operations is repeated. In normal operation, the time required for the discharge of tube El and condenser l0, (a. to c) is only a small fraction of the time required to relieve the vnegative charge upon grid 3 (c to a'). Since the time from a to b requires, say, one unit of time, from b to c ten units of time, and from c to a' 10,000 units of time, the time vaxis of the curves has been distorted to more clearly show in the curves the operating conditions in the circuits.

Such variations in tube capacities as grid-tocathode and grid-to-plate have substantially no eii'ect upon the calibration of my device inasmuch as the potential of grid 3 remains essentially unchanged during the part of the cycle a to b in which condenser is charged. A given change in potential of point 2| results in a given quantity of electricity being fed into condenser 20. This quantity or charge of electricity fed high impedance circuits.

aoeaaee into condenser 20 distributes itself, as the potential oi point 2l drops to zero (c), among, the capacities between the grid and grounded parts of the circuit and tube. Whether the tube capacities are large or small, this quantity of electricity fed into the grid parts is fixed only by the swing of contact 2l and size of condenser 20 and requires a deiinite interval of time to relieve that charge through Rx. The frequency of relaxation further has been found to be little aiiected by changes in power supply voltages.

Practically all of the time the negative charge on the grid is being dissipated through resistor Rx the plate current of tube l is zero, which means that no current due to residual gas in tube i may iiow to the grid during this time. Further, to minimize thermionic space current to the grid from the heater, the heater temperature may be lowered, and the heater maintained at a potential positive with respect to the grid. Good results have been obtained by using for tubes l and Il tube types commercially known as the 2A6 and the 885 and with condensers i9 and 20 respectively of about .l micro-fared and 1 micro-micro-farad, and with resistors 5, T, lG and i6 respectively 20,000 ohms, 100,000 ohms, 250,000 ohms, and 10,000 ohms, and with inductance il, 8 milli-henries. A circuit thus constructed has been found useful in measuring small phototube currents, resistance leakage currents in insulators, and other minute currents in My device may conveniently be employed in measuring the leakage resistance of condensers by open circuiting terminals 9 and I0 and connecting the condensers to be tested in place of condenser 20, in which case meter I5 may be calibrated directly in units of resistance per unit of capacity.

While I have indicated one preferred embodiment of my invention, it will be apparent to those skilled in the art that my invention is by 4no means limited to the exact form illustrated, but that many variations may be made in the particular structure and connections used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

r claim:l

l. A current measuring device comprising an electron discharge device with an anode, a grid, and a cathode, the anode and cathode being connected in series with a resistance through an anode voltage source, a current meter, a resistor, a grid controlled gas discharge tube with its anode and cathode connected in series with said meter andsaid resistor across said source, the grid and cathode of said gas tube connected respectively to spaced points on said resistance, a condenser connected between the first-mentioned grid and an intermediate point on said resistor, and means for changing the potential of the iirst-mentioned grid.

2. A current measuring device comprising an electron discharge tube with an anode, a grid, and a cathode, a coupling impedance in series with a voltage source connected across said anode and said cathode, two input test terminals, a `condenser`, the grid of the tube being connected to one of said test terminals and to one plate of said condenser, a gas discharge device with control grid and cathode coupled across said impedance, means for impressing the transitory voltage, caused by gaseous discharges, on the grid of said tube including a. connection from the other plate of said condenser to the output circuit of the gas discharge device and means in circuit with said gas discharge device for indicating the frequency of gaseous discharges.

'3. In combinationfan electron discharge tube with an anode, a grid, and a cathode, a voltage source and a coupling impedance connected in the output circuit of said tube, means responsive tol remove said charges onthe grid comprising two test terminals, one terminal being connected to said grid and the other terminal being connected to a point of a potential equal to or higher than the potential of said cathode and means responsive to `current in said impedance to indicate the periodicity of the negative charges on the grid.

4. 'I'he combination of an Aelectron discharge device with a control element and a resistor in its output circuit, a gas triode with a control grid and having an nductance and capacity in series in the ouput circuit of the triode, a connection between the grid of said triode and a rst point on said resistor, a connection between the cath-V ode of the triode and a second point on said resistor more negative than said first point, a condenser connected between the output circuit of said triode and the control element of said device, and one test terminal connected to the control element of said device and another test terminal connected to a point in theoutput circuit or said discharge device. l Y

5. In combination an electron discharge device with a control element and a resistor in the output circuit of said device, a gas triode with a control grid and an oscillatory circuit connected in the output o! said triode, connections from the cathode and control grid of said triode to spaced points on said resistor, a condenser connected 

